crypttab - Configuration for encrypted block devices
The /etc/crypttab file describes encrypted block devices that are set up during system boot.
Empty lines and lines starting with the "#" character are ignored. Each of the remaining lines describes one encrypted block device. Fields are delimited by white space.
Each line is in the form
volume-name encrypted-device key-file options
The first two fields are mandatory, the remaining two are optional.
Setting up encrypted block devices using this file supports four encryption modes: LUKS, TrueCrypt, BitLocker and plain. See cryptsetup(8) for more information about each mode. When no mode is specified in the options field and the block device contains a LUKS signature, it is opened as a LUKS device; otherwise, it is assumed to be in raw dm-crypt (plain mode) format.
The four fields of /etc/crypttab are defined as follows:
If the specified key file path refers to an AF_UNIX stream socket in the file system, the key is acquired by connecting to the socket and reading it from the connection. This allows the implementation of a service to provide key information dynamically, at the moment when it is needed. For details see below.
Six different mechanisms for acquiring the decryption key or passphrase unlocking the encrypted volume are supported. Specifically:
For the latter five mechanisms the source for the key material used for unlocking the volume is primarily configured in the third field of each /etc/crypttab line, but may also configured in /etc/cryptsetup-keys.d/ and /run/cryptsetup-keys.d/ (see above) or in the LUKS2 JSON token header (in case of the latter three). Use the systemd-cryptenroll(1) tool to enroll PKCS#11, FIDO2 and TPM2 devices in LUKS2 volumes.
The following options may be used in the fourth field of each line:
Optionally, the path may be followed by ":" and an /etc/fstab device specification (e.g. starting with "UUID=" or similar); in which case, the path is relative to the device file system root. The device gets mounted automatically for LUKS device activation duration only.
Hint: if this device is used for a mount point that is specified in fstab(5), the _netdev option should also be used for the mount point. Otherwise, a dependency loop might be created where the mount point will be pulled in by local-fs.target, while the service to configure the network is usually only started after the local file system has been mounted.
This requires kernel 4.0 or newer.
This requires kernel 4.0 or newer.
This requires kernel 5.9 or newer.
This requires kernel 5.9 or newer.
This option is only relevant for plain devices.
WARNING: Using the swap option will destroy the contents of the named partition during every boot, so make sure the underlying block device is specified correctly.
When this mode is used, the passphrase is read from the key file given in the third field. Only the first line of this file is read, excluding the new line character.
Note that the TrueCrypt format uses both passphrase and key files to derive a password for the volume. Therefore, the passphrase and all key files need to be provided. Use tcrypt-keyfile= to provide the absolute path to all key files. When using an empty passphrase in combination with one or more key files, use "/dev/null" as the password file in the third field.
This will map the hidden volume that is inside of the volume provided in the second field. Please note that there is no protection for the hidden volume if the outer volume is mounted instead. See cryptsetup(8) for more information on this limitation.
See the entry for tcrypt on the behavior of the passphrase and key files when using TrueCrypt encryption mode.
WARNING: Using the tmp option will destroy the contents of the named partition during every boot, so make sure the underlying block device is specified correctly.
If enabled, the typed characters are echoed literally. If disabled, the typed characters are not echoed in any form, the user will not get feedback on their input. If set to "masked", an asterisk ("*") is echoed for each character typed. Regardless of which mode is chosen, if the user hits the tabulator key ("↹") at any time, or the backspace key ("⌫") before any other data has been entered, then echo is turned off.
If specified as "auto" the volume must be of type LUKS2 and must carry PKCS#11 security token metadata in its LUKS2 JSON token section. In this mode the URI and the encrypted key are automatically read from the LUKS2 JSON token header. Use systemd-cryptenroll(1) as simple tool for enrolling PKCS#11 security tokens or smartcards in a way compatible with "auto". In this mode the third column of the line should remain empty (that is, specified as "-").
The specified URI can refer directly to a private RSA key stored on a token or alternatively just to a slot or token, in which case a search for a suitable private RSA key will be performed. In this case if multiple suitable objects are found the token is refused. The encrypted key configured in the third column of the line is passed as is (i.e. in binary form, unprocessed) to RSA decryption. The resulting decrypted key is then Base64 encoded before it is used to unlock the LUKS volume.
Use systemd-cryptenroll --pkcs11-token-uri=list to list all suitable PKCS#11 security tokens currently plugged in, along with their URIs.
Note that many newer security tokens that may be used as PKCS#11 security token typically also implement the newer and simpler FIDO2 standard. Consider using fido2-device= (described below) to enroll it via FIDO2 instead. Note that a security token enrolled via PKCS#11 cannot be used to unlock the volume via FIDO2, unless also enrolled via FIDO2, and vice versa.
If specified as "auto" the FIDO2 token device is automatically discovered, as it is plugged in.
FIDO2 volume unlocking requires a client ID hash (CID) to be configured via fido2-cid= (see below) and a key to pass to the security token's HMAC functionality (configured in the line's third column) to operate. If not configured and the volume is of type LUKS2, the CID and the key are read from LUKS2 JSON token metadata instead. Use systemd-cryptenroll(1) as simple tool for enrolling FIDO2 security tokens, compatible with this automatic mode, which is only available for LUKS2 volumes.
Use systemd-cryptenroll --fido2-device=list to list all suitable FIDO2 security tokens currently plugged in, along with their device nodes.
This option implements the following mechanism: the configured key is hashed via they HMAC keyed hash function the FIDO2 device implements, keyed by a secret key embedded on the device. The resulting hash value is Base64 encoded and used to unlock the LUKS2 volume. As it should not be possible to extract the secret from the hardware token, it should not be possible to retrieve the hashed key given the configured key — without possessing the hardware token.
Note that many security tokens that implement FIDO2 also implement PKCS#11, suitable for unlocking volumes via the pkcs11-uri= option described above. Typically the newer, simpler FIDO2 standard is preferable.
Use tpm2-pcrs= (see below) to configure the set of TPM2 PCRs to bind the volume unlocking to. Use systemd-cryptenroll(1) as simple tool for enrolling TPM2 security chips in LUKS2 volumes.
If specified as "auto" the TPM2 device is automatically discovered. Use systemd-cryptenroll --tpm2-device=list to list all suitable TPM2 devices currently available, along with their device nodes.
This option implements the following mechanism: when enrolling a TPM2 device via systemd-cryptenroll on a LUKS2 volume, a randomized key unlocking the volume is generated on the host and loaded into the TPM2 chip where it is encrypted with an asymmetric "primary" key pair derived from the TPM2's internal "seed" key. Neither the seed key nor the primary key are permitted to ever leave the TPM2 chip — however, the now encrypted randomized key may. It is saved in the LUKS2 volume JSON token header. When unlocking the encrypted volume, the primary key pair is generated on the TPM2 chip again (which works as long as the chip's seed key is correctly maintained by the TPM2 chip), which is then used to decrypt (on the TPM2 chip) the encrypted key from the LUKS2 volume JSON token header saved there during enrollment. The resulting decrypted key is then used to unlock the volume. When the randomized key is encrypted the current values of the selected PCRs (see below) are included in the operation, so that different PCR state results in different encrypted keys and the decrypted key can only be recovered if the same PCR state is reproduced.
Although it's not necessary to mark the mount entry for the root file system with x-initrd.mount, x-initrd.attach is still recommended with the encrypted block device containing the root file system as otherwise systemd will attempt to detach the device during the regular system shutdown while it's still in use. With this option the device will still be detached but later after the root file system is unmounted.
All other encrypted block devices that contain file systems mounted in the initrd should use this option.
At early boot and when the system manager configuration is reloaded, this file is translated into native systemd units by systemd-cryptsetup-generator(8).
If the key file path (as specified in the third column of /etc/crypttab entries, see above) refers to an AF_UNIX stream socket in the file system, the key is acquired by connecting to the socket and reading the key from the connection. The connection is made from an AF_UNIX socket name in the abstract namespace, see unix(7) for details. The source socket name is chosen according the following format:
NUL RANDOM /cryptsetup/ VOLUME
In other words: a NUL byte (as required for abstract namespace sockets), followed by a random string (consisting of alphanumeric characters only), followed by the literal string "/cryptsetup/", followed by the name of the volume to acquire they key for. For example, for the volume "myvol":
Services listening on the AF_UNIX stream socket may query the source socket name with getpeername(2), and use this to determine which key to send, allowing a single listening socket to serve keys for multiple volumes. If the PKCS#11 logic is used (see above), the socket source name is picked in similar fashion, except that the literal string "/cryptsetup-pkcs11/" is used. And similarly for FIDO2 ("/cryptsetup-fido2/") and TPM2 ("/cryptsetup-tpm2/"). A diffent path component is used so that services providing key material know that the secret key was not requested directly, but instead an encrypted key that will be decrypted via the PKCS#11/FIDO2/TPM2 logic to acquire the final secret key.
Example 1. /etc/crypttab example
Set up four encrypted block devices. One using LUKS for normal storage, another one for usage as a swap device and two TrueCrypt volumes. For the fourth device, the option string is interpreted as two options "cipher=xchacha12,aes-adiantum-plain64", "keyfile-timeout=10s".
luks UUID=2505567a-9e27-4efe-a4d5-15ad146c258b swap /dev/sda7 /dev/urandom swap truecrypt /dev/sda2 /etc/container_password tcrypt hidden /mnt/tc_hidden /dev/null tcrypt-hidden,tcrypt-keyfile=/etc/keyfile external /dev/sda3 keyfile:LABEL=keydev keyfile-timeout=10s,cipher=xchacha12\,aes-adiantum-plain64
Example 2. Yubikey-based PKCS#11 Volume Unlocking Example
The PKCS#11 logic allows hooking up any compatible security token that is capable of storing RSA decryption keys for unlocking an encrypted volume. Here's an example how to set up a Yubikey security token for this purpose on a LUKS2 volume, using ykmap(1) from the yubikey-manager project to initialize the token and systemd-cryptenroll(1) to add it in the LUKS2 volume:
# SPDX-License-Identifier: MIT-0 # Destroy any old key on the Yubikey (careful!) ykman piv reset # Generate a new private/public key pair on the device, store the public key in # 'pubkey.pem'. ykman piv generate-key -a RSA2048 9d pubkey.pem # Create a self-signed certificate from this public key, and store it on the # device. The "subject" should be an arbitrary user-chosen string to identify # the token with. ykman piv generate-certificate --subject "Knobelei" 9d pubkey.pem # We don't need the public key anymore, let's remove it. Since it is not # security sensitive we just do a regular "rm" here. rm pubkey.pem # Enroll the freshly initialized security token in the LUKS2 volume. Replace # /dev/sdXn by the partition to use (e.g. /dev/sda1). sudo systemd-cryptenroll --pkcs11-token-uri=auto /dev/sdXn # Test: Let's run systemd-cryptsetup to test if this all worked. sudo /usr/lib/systemd/systemd-cryptsetup attach mytest /dev/sdXn - pkcs11-uri=auto # If that worked, let's now add the same line persistently to /etc/crypttab, # for the future. sudo bash -c 'echo "mytest /dev/sdXn - pkcs11-uri=auto" >> /etc/crypttab'
A few notes on the above:
Example 3. FIDO2 Volume Unlocking Example
The FIDO2 logic allows using any compatible FIDO2 security token that implements the "hmac-secret" extension for unlocking an encrypted volume. Here's an example how to set up a FIDO2 security token for this purpose for a LUKS2 volume, using systemd-cryptenroll(1):
# SPDX-License-Identifier: MIT-0 # Enroll the security token in the LUKS2 volume. Replace /dev/sdXn by the # partition to use (e.g. /dev/sda1). sudo systemd-cryptenroll --fido2-device=auto /dev/sdXn # Test: Let's run systemd-cryptsetup to test if this worked. sudo /usr/lib/systemd/systemd-cryptsetup attach mytest /dev/sdXn - fido2-device=auto # If that worked, let's now add the same line persistently to /etc/crypttab, # for the future. sudo bash -c 'echo "mytest /dev/sdXn - fido2-device=auto" >> /etc/crypttab'
Example 4. TPM2 Volume Unlocking Example
The TPM2 logic allows using any TPM2 chip supported by the Linux kernel for unlocking an encrypted volume. Here's an example how to set up a TPM2 chip for this purpose for a LUKS2 volume, using systemd-cryptenroll(1):
# SPDX-License-Identifier: MIT-0 # Enroll the TPM2 security chip in the LUKS2 volume, and bind it to PCR 7 # only. Replace /dev/sdXn by the partition to use (e.g. /dev/sda1). sudo systemd-cryptenroll --tpm2-device=auto --tpm2-pcrs=7 /dev/sdXn # Test: Let's run systemd-cryptsetup to test if this worked. sudo /usr/lib/systemd/systemd-cryptsetup attach mytest /dev/sdXn - tpm2-device=auto # If that worked, let's now add the same line persistently to /etc/crypttab, # for the future. sudo bash -c 'echo "mytest /dev/sdXn - tpm2-device=auto" >> /etc/crypttab'
- RFC7512 PKCS#11 URI
- see documentation